Data storage media are commonly used for storage and retrieval of data, and come in many forms, such as magnetic tape, magnetic disks, optical tape, optical disks, holographic disks or cards, and the like. Magnetic tape media remains economical for storing large amounts of data. For example, magnetic tape cartridges, or large spools of magnetic tape, are often used to back up data in large computing centers. Magnetic tape cartridges also find application in the backup of data stored on smaller computers such as desktop or notebook computers.
In magnetic media, data is typically stored as magnetic signals that are magnetically recorded on the medium surface. The data stored on the medium is typically organized along “data tracks,” and transducer heads are positioned relative to the data tracks to write data to the tracks or read data from the tracks. A typical magnetic storage medium, such as magnetic tape, typically includes several data tracks in a data band. Optical media, holographic media, and other media formats can also make use of data tracks.
Servo patterns refer to signals or other recorded marks on the medium that are used for tracking purposes. In other words, servo patterns are recorded on the medium to provide reference points relative to the data tracks. A servo controller interprets detected servo patterns and generates position error signals. The position error signals are used to adjust the lateral distance of the transducer head relative to the data tracks so that the transducer head is properly positioned along the data tracks for effective reading and/or writing of the data to the data tracks.
With some data storage media, such as magnetic tape, the servo patterns are stored in specialized tracks on the medium, called “servo tracks.” Servo tracks serve as references for the servo controller. Servo tracks typically hold no data except for information that is useful to the servo controller to identify positioning of a transducer head relative to the surface of the medium. A plurality of servo tracks may be defined in a servo band. Some magnetic media include a plurality of servo bands, with data tracks being located between the servo bands.
The servo patterns recorded in the servo tracks may be sensed by one or more servo heads. For example, servo heads may be dedicated heads that read only servo patterns in the servo tracks. Alternatively, servo heads may be integrated with a read/write head. In any case, once a particular servo track is located by the servo head, one or more data tracks can be located on the medium according to the data track's known displacement from the servo track. The servo controller receives detected servo signals from the servo heads, and generates position error signals, which are used to adjust positioning of a read/write head relative to the data tracks.
Time-based servo techniques refer to servo techniques that make use of time variables. Time-based servo techniques are particularly effective for magnetic tape, which typically feeds past transducer heads at a constant velocity. For example, N-shaped servo markings, servo markings such as “<<< >>>” or “//// \\\\,” or the like, have been developed for time-based servo techniques. Such markings are typically formed in a servo band of the magnetic tape and may define a plurality of servo tracks in the servo band.
When time-based servo techniques are used, the time offset between detection of two or more servo marks can be translated into a position error signal, which defines a lateral distance of the transducer head relative to a data track. For example, given a constant velocity of magnetic tape formed with marking “/ \”, the time between detection of “/” and “\” becomes larger when the servo head is positioned towards the bottom of marking “/ \” and smaller if the servo head positioned towards the top of marking “/ \”. Given a constant velocity of magnetic tape, a defined time period between detected servo signals may correspond to a center of marking “/ \”. By locating the center of marking “/ \”, a known distance between the center of the servo track and the data tracks can be identified. Different servo tracks may be defined by different traverse locations along the marking “/ \”.
A servo head that is able to read and/or write full band time-based servo marks includes servo gaps oriented parallel to the servo markings. A full band servo verify head is used to read back the entire width of the servo markings to detect flaws in the written servo signal. In time-based servo systems, part or all of the servo signal is recorded at an angle non-parallel to both the down tape and transverse directions of tape motion. The verification process for time-based servo markings requires servo read gaps to be oriented parallel to the directions of the servo markings. This is accomplished by orienting individual ferrite composite servo head cores using precision assembly and machining. The resultant servo head has high labor content and low yield, which makes this type of servo verify head expensive to manufacture.